Slip is a relative measure of the difference between rotary field speed and rotor speed and hence is an important operational parameter for induction motors.
Arrangements are known which measure the rotational speed (Trenkler, G. "Die elektrische Messung von Drehzahl und Winkelgeschwindigkeit" , Verlag E. Geyer, Bad Worishofen, 1974, in particular pages 36 and 37) so that the slip can be calculated. For an inverter supply of a motor, it is known to calculate the slip by means of a slip computer from the motor current, motor flux, and ignition angle (German Published Unexamined Patent Application 35 23 619). Also known are computer-assisted methods of estimating slip (Hillenbrand, F., "Identification of linear time-invariant systems and their application to induction motors" , Dissertation, TU Berlin 1982, pages 149-157). Another known method for measuring slip is the provision of a separate measuring motor (Swiss Patent 449,770) having an air gap in which Hall probes are disposed. Further, to determine the rotational speed in universal motors utilizing effective harmonics, application of the Fast Fourier Transform is known (French Patent 25 55 317).
The above known arrangements and methods require speed sensors or are suitable only for certain motor types and circuit variants, or else they require precise prior knowledge concerning the measured object. The cost involved in the evaluation electronics and software is usually considerable.
To provide rotational speed determination, a measuring arrangement was proposed (in German Patent Application 37 11 976.1 of Apr. 9, 1987), which determines the time responses of the voltages at the electrical connections and/or the currents through the electrical connections of the motor.
The problem presented by the prior art is to provide an arrangement which permits in a universal and simple manner the measurement of the slip of electric induction motors.